Coin validators which can discriminate between coins of different denominations are well known and one example is described in our UK Patent No. 2 169 429. This coin validator includes a coin rundown path along which coin pass edgewise through a primary sensing station at which coils perform a series of primary inductive tests on the coin to develop coin parameter signals which are indicative of a material and metallic content of the coin under test. The coin parameter signals are compared by a microprocessor with stored data corresponding to the plurality of coins of different denominations. If the coin is found to be acceptable as a result of the primary validation testings, the microprocessor operates an accept gate so that the coin is directed to an accept path. Otherwise, the accept gate remains inoperative and causes the coin to pass to a reject path.
Whilst the primary validation testing can discriminate adequately between true coins of different denominations and fraudulent coins, further auxiliary validation testing is carried out in order to check whether an attempt is being made to defraud the validator by a non-standard fraudulent operation. In the past, frauds have been attempted by lowering a true coin attached to a length of string into the validator so as to trigger a successful outcome during primary validation testing, and then to insert a coin of lower value. The true coin on a string is then withdrawn from the validator for re-use.
In the past, to overcome this problem, an accept sensor has been provided in the accept path and the validator generates a credit code indicating the presence of a true coin, only if the true coin passes the accept sensor within a predetermined time range subsequent to successful primary validation testing. Another possible fraud involves lowering the coin on a string into the vicinity of the further sensor in the coin accept path. However, the timing at which the coin on the string arrives at the sensor usually differs from that for a true coin so that an analysis of the timing relationship of the signals from the primary testing and the auxiliary testing permits discrimination against fraudulent operation performed in this manner.
A problem associated with the auxiliary testing is that it limits the rate at which the primary validation can be carried out on successive coins fed into the validator. Thus, when coins are fed into the validator at a rate that exceeds a given threshold, the primary validation testing carried out on a first coin is followed by auxiliary validation tests thereon, and the auxiliary validation tests may not be completed by the time that the primary validation testing is carried out on the next coin. In order to avoid any risk of fraud, the validator is programmed to reject the second coin even though it may actually be a true coin.